1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with a unique control rod design in which the worth of the control rod can be changed approximately uniformly in the axial direction of the rod.
2. Description of the Prior Art
In a typical nuclear reactor, the reactor core includes a large number of fuel assemblies each of which is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending longitudinally between the nozzles and a plurality of transverse support grids axially spaced along and attached to the guide thimbles. Also, each fuel assembly is composed of a plurality of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles and supported by the transverse grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
Since the rate of heat generation in the reactor core is proportional to the nuclear fission rate, and this, in turn, is determined by the neutron flux in the core, control of heat generation at reactor start-up, during its operation and at shutdown is achieved by varying the neutron flux. Generally, this is done by absorbing excess neutrons using control rods which contain neutron absorbing material. The guide thimbles, in addition to being structural elements of the fuel assembly, also provide channels for insertion of the neutron absorber control rods within the reactor core. The level of neutron flux and thus the heat output of the core is normally regulated by the movement of the control rods into and from the guide thimbles.
One common arrangement utilizing control rods in association with a fuel assembly can be seen in U.S. Pat. No. 4,326,919 to Hill and assigned to the assignee of the present invention. This patent shows an array of control rods supported at their upper ends by a spider assembly, which in turn is connected to a control rod drive mechanism that vertically raises and lowers (referred to as a stepping action) the control rods into and out of the hollow guide thimbles of the fuel assembly. The typical construction of the control rod used in such an arrangement is in the form of an elongated metallic cladding tube having a neutron absorbing material disposed within the tube and with end plugs at opposite ends thereof for sealing the absorber material within the tube. Generally, the neutron absorbing material is in the form of a stack of closely packed ceramic or metallic pellets which only partially fill the tube, leaving a void space or axial gap between the top of the pellets and the upper end plug in defining a plenum chamber for receiving gases generated during the control operation. A coil spring is disposed within this plenum chamber and held in a state of compression between the upper end plug and the top pellet so as to maintain the stack of pellets in their closely packed arrangement during stepping of the control rod.
Thus, control rods affect reactivity by changing direct neutron absorption. Control rods are used for fast reactivity control. A chemical shim such as boric acid dissolved in the coolant is used to control long term reactivity changes. More uniformly distributed throughout the core, the boron solution leads to a more uniform power distribution and fuel depletion than do control rods. The concentration of boron is normally decreased with core age to compensate for fuel depletion and fission product buildup.
The buildup of fission products, such as Xenon-135, reduces reactivity by parasitically absorbing neutrons, thereby decreasing thermal utilization. The Xenon-135 (hereafter referred to as just "xenon") is removed by neutron absorption or by decay. Upon a reduction in core power (such as during load follow, which is a reduction in reactor power in response to a reduction in power demand), fewer thermal neutrons are available to remove the xenon and therefore the concentration of xenon in the core increases.
This increase in xenon concentration which accompanies a reduction in core reactivity is usually compensated for by either decreasing the concentration of boron dissolved in the core coolant or by withdrawing the control rods from the core. However, both of these methods have drawbacks. Changing the boron concentration requires the processing of coolant (i.e., water) which is difficult and not desired by the utility, especially towards the end of core life (EOL). Removal of control rods means that the core's return to power capability is reduced and peaking factors are increased.
The usual solution to this problem is to have several banks of reduced worth rods (i.e., grey rods) in the core at full power which are available for removal at reduced power to compensate for the xenon buildup. The drawback of this procedure is that moving these banks changes the axial offset and increases peaking factors. Also, because these reduced worth banks are in the core at power, shutdown margin can be affected.
Consequently, a need exists for a different approach to xenon compensation, one which will effectively resolve the problem of xenon buildup during load follow, but which will not raise a host of new problems in the process.